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          <h1 class="post-title" itemprop="name headline">Java的四种引用方式</h1>
        

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        <p>java语言提供了4种引用类型：强引用（StrongReference）、软引用(SoftReference)、弱引用（WeakReference）和虚引用（PhantomReference），与引用密切相关的，还有一个引用队列ReferenceQueue。<br>Java中提供这四种引用类型主要有两个目的：</p>
<ul>
<li>可以让程序员通过代码的方式决定某些对象的生命周期</li>
<li>有利于JVM进行垃圾回收</li>
</ul>
<h4 id="强引用（StrongReference）"><a href="#强引用（StrongReference）" class="headerlink" title="强引用（StrongReference）"></a>强引用（StrongReference）</h4><p>是指创建一个对象并把这个对象赋给一个引用变量。强引用不会被GC回收，并且在<code>java.lang.ref</code>里也没有实际的对应类型，平时工作接触的最多的就是强引用。比如：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">Object object = <span class="keyword">new</span> Object();</span><br><span class="line">String str = <span class="string">"hello"</span>;</span><br></pre></td></tr></table></figure></p>
<p>只要某个对象有强引用与之关联，JVM必定不会回收这个对象，即使在内存不足的情况下，JVM宁愿抛出<code>OutOfMemory</code>错误也不会回收这种对象。<br>如果想中断强引用和某个对象之间的关联，可以显示地将引用赋值为<code>null</code>，这样一来的话，JVM在合适的时间就会回收该对象。</p>
<h4 id="软引用（SoftReference）"><a href="#软引用（SoftReference）" class="headerlink" title="软引用（SoftReference）"></a>软引用（SoftReference）</h4><p>如果一个对象具有软引用，内存空间足够，垃圾回收器就不会回收它；如果内存空间不足了，就会回收这些对象的内存。只要垃圾回收器没有回收它，该对象就可以被程序使用。</p>
<p>软引用可用来实现内存敏感的高速缓存，比如网页缓存、图片缓存等。使用软引用能防止内存泄露，增强程序的健壮性。<br>软引用可以和一个引用队列（<code>ReferenceQueue</code>）联合使用，如果软引用所引用的对象被JVM回收，这个软引用就会被加入到与之关联的引用队列中。下面是一个使用示例：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">MyObject aRef = <span class="keyword">new</span>  MyObject();  </span><br><span class="line">SoftReference aSoftRef=<span class="keyword">new</span> SoftReference(aRef);</span><br></pre></td></tr></table></figure></p>
<p>此时，对于这个MyObject对象，有两个引用路径，一个是来自<code>SoftReference</code>对象的软引用，一个来自变量aRefe的强引用，所以这个MyObject对象是强可及对象。<br>我们可以通过对aRef变量显示的赋值为null来结束aRef对这个MyObject实例的强引用：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">aRef = <span class="keyword">null</span>;</span><br></pre></td></tr></table></figure></p>
<p>此后，这个MyObject对象成为了软引用对象。如果垃圾收集线程进行内存垃圾收集，并不会因为有一个<code>SoftReference</code>对该对象的引用而始终保留该对象。</p>
<p>如果在创建<code>SoftReference</code>对象的时候，使用了一个<code>ReferenceQueue</code>对象作为参数提供给<code>SoftReference</code>的构造方法，那么当这个<code>SoftReference</code>对象所软引用的aMyOhject被垃圾收集器回收的同时，这个<code>SoftReference</code>对象会被放入<code>ReferenceQueue</code>。<br>也就是说，<code>ReferenceQueue</code>中保存的对象是已经失去了它所软引用的对象的<code>Reference</code>对象。</p>
<p><code>ReferenceQueue</code>是一个队列，当我们调用它的<code>poll()</code>方法的时候，如果这个队列中不是空队列，那么将返回队列前面的那个<code>Reference</code>对象；如果队列为空，将返回一个<code>null</code>。利用这个方法，我们可以检查哪个<code>SoftReference</code>所软引用的对象已经被回收。</p>
<h4 id="弱引用（WeakReference）"><a href="#弱引用（WeakReference）" class="headerlink" title="弱引用（WeakReference）"></a>弱引用（WeakReference）</h4><p>弱引用也是用来描述非必需对象的，当JVM进行垃圾回收时，无论内存是否充足，都会回收被弱引用关联的对象。在java中，用<code>java.lang.ref.WeakReference</code>类来表示。下面是使用示例：<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">test</span> </span>&#123;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;  </span><br><span class="line">        WeakReference&lt;People&gt; reference=<span class="keyword">new</span> WeakReference&lt;People&gt;(<span class="keyword">new</span> People(<span class="string">"zhouqian"</span>,<span class="number">20</span>));  </span><br><span class="line">        System.out.println(reference.get());  </span><br><span class="line">        System.gc();<span class="comment">//通知GVM回收资源  </span></span><br><span class="line">        System.out.println(reference.get());  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;  </span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">People</span></span>&#123;  </span><br><span class="line">    <span class="keyword">public</span> String name;  </span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">int</span> age;  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">People</span><span class="params">(String name,<span class="keyword">int</span> age)</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">this</span>.name=name;  </span><br><span class="line">        <span class="keyword">this</span>.age=age;  </span><br><span class="line">    &#125;  </span><br><span class="line">    <span class="meta">@Override</span>  </span><br><span class="line">    <span class="function"><span class="keyword">public</span> String <span class="title">toString</span><span class="params">()</span> </span>&#123;  </span><br><span class="line">        <span class="keyword">return</span> <span class="string">"[name:"</span>+name+<span class="string">",age:"</span>+age+<span class="string">"]"</span>;  </span><br><span class="line">    &#125;  </span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure></p>
<p>输出结果为：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">hello</span><br><span class="line">null</span><br></pre></td></tr></table></figure></p>
<p>第二个输出结果是null，这说明只要JVM进行垃圾回收，被弱引用关联的对象必定会被回收掉。不过要注意的是，这里所说的被弱引用关联的对象是指只有弱引用与之关联，如果存在强引用同时与之关联，则进行垃圾回收时也不会回收该对象（软引用也是如此）。<br>弱引用可以和一个引用队列（<code>ReferenceQueue</code>）联合使用，如果弱引用所引用的对象被JVM回收，这个软引用就会被加入到与之关联的引用队列中。</p>
<h4 id="虚引用（PhantomReference）"><a href="#虚引用（PhantomReference）" class="headerlink" title="虚引用（PhantomReference）"></a>虚引用（PhantomReference）</h4><p>虚引用和前面的软引用、弱引用不同，它并不影响对象的生命周期。在java中用<code>java.lang.ref.PhantomReference</code>类表示。如果一个对象与虚引用关联，则跟没有引用与之关联一样，在任何时候都可能被垃圾回收器回收。</p>
<p>要注意的是，虚引用必须和引用队列关联使用，当垃圾回收器准备回收一个对象时，如果发现它还有虚引用，就会把这个虚引用加入到与之关联的引用队列中。程序可以通过判断引用队列中是否已经加入了虚引用，来了解被引用的对象是否将要被垃圾回收。如果程序发现某个虚引用已经被加入到引用队列，那么就可以在所引用的对象的内存被回收之前采取必要的行动。</p>
<h4 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h4><p>对于强引用，我们平时在编写代码时经常会用到。而对于其他三种类型的引用，使用得最多的就是软引用和弱引用，这2种既有相似之处又有区别。它们都是用来描述非必需对象的，但是<strong>被软引用关联的对象只有在内存不足时才会被回收，而被弱引用关联的对象在JVM进行垃圾回收时总会被回收。</strong><br>针对上面的特性，<strong>软引用适合用来进行缓存，当内存不够时能让JVM回收内存，弱引用能用来在回调函数中防止内存泄露。</strong>因为回调函数往往是匿名内部类，隐式保存有对外部类的引用，所以如果回调函数是在另一个线程里面被回调，而这时如果需要回收外部类，那么就会内存泄露，因为匿名内部类保存有对外部类的强引用。</p>
<p>为了增强程序的健壮性，减少OOM异常，平时要保持良好的编程习惯：</p>
<ul>
<li>尽早释放无用对象的引用。大多数程序员在使用临时变量的时候，都是让引用变量在退出活动域（scope）后，自动设置为<code>null</code>。如果程序允许，尽早将不用的引用对象赋为<code>null</code>，这样可以加速GC的工作</li>
<li>尽量少用<code>finalize</code>函数。<code>finalize</code>函数是Java提供给程序员一个释放对象或资源的机会。但是，它会加大GC的工作量，因此尽量少采用<code>finalize</code>方式回收资源</li>
<li>如果需要使用经常使用的图片，可以使用soft应用类型。它可以尽可能将图片保存在内存中，供程序调用，而不引起OutOfMemory</li>
<li>注意集合数据类型，包括数组，树，图，链表等数据结构，这些数据结构对GC来说，回收更为复杂。另外，注意一些全局的变量，以及一些静态变量。这些变量往往容易引起悬挂对象（danglingreference），造成内存浪费</li>
<li>当程序有一定的等待时间，程序员可以手动执行<code>System.gc()</code>，通知GC运行，但是Java语言规范并不保证GC一定会执行。使用增量式GC可以缩短Java程序的暂停时间</li>
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